Sulfur dioxide may be used for producing sulfuric acid or liquid sulfur trioxide and generally is produced by calcining/smelting sulfur-containing ores or by combustion of elementary sulfur. The combustion generally is effected with atmospheric air, but air enriched with oxygen or even pure oxygen are used as well. For cost reasons, however, the use of pure oxygen for the combustion of sulfur mostly may not be expedient. Nowadays, sulfur itself is used almost exclusively in liquid form and in general is supplied as a liquid and stored temporarily. The liquid sulfur is supplied to the combustion furnace with temperatures of 140 to 150° C., at which its viscosity is low enough to provide for injection via nozzles. In order to optimize the combustion, the liquid sulfur is atomized in the furnace and is thoroughly mixed with the combustion air.
The combustion of sulfur requires equal molar quantities of sulfur and oxygen. With ambient air, which contains 20.95 vol-% of O2, an SO2 gas with a maximum of 20.5 vol-% of SO2 can theoretically be obtained with a stoichiometric combustion of sulfur. To ensure a complete combustion of sulfur, an excess of air is usually supplied. Problems resulting from unburnt sulfur, which is condensed and deposited in colder parts of the plant, thus can be avoided. The hyperstoichiometric combustion is described for instance in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1994, vol. A25, p. 574 f. The combustion is effected in a horizontally arranged cylindrical furnace, which has a refractory lining and on its end face includes a centrally arranged system of burners. The liquid sulfur is atomized and mixed with the combustion air and burns at temperatures of 600 to 1,600° C. in dependence on the desired sulfur dioxide concentration. Subsequent to the sulfur furnace, a waste heat boiler is provided, before the gas is supplied to a sulfuric acid contact plant. The converter of the contact plant generally employs initial concentrations of sulfur dioxide of 10 to 12 vol-%, which must possibly be adjusted by means of further equipment.
EP 0 762 990 B1 also describes the hyperstoichiometric combustion of sulfur.
A method and an apparatus for the combustion of liquid sulfur into sulfur dioxide in a burner furnace is described in U.S. Pat. No. 3,879,530. The liquid sulfur and the primary combustion-supporting air are injected concurrently in a first part of the combustion furnace. A secondary combustion-supporting gas is introduced further downstream of the furnace in the combustion chamber prior to the outlet of the burned gases containing sulfur dioxide into an adjoining boiler.
At combustion temperatures above 1,100° C., the formation of nitrogen oxides (NOx) increases strongly, even if less free oxygen is available for the formation of nitrogen oxides due to the higher sulfur dioxide concentration. Only above a sulfur dioxide concentration of 18 vol-% does the formation of NOx decrease again due to the lack of oxygen. The formation of nitrogen oxides therefore limits the preheating of the combustion gases in conventional sulfur combustion systems, as the combustion temperature increases. This impairs the economy of the plants.
For producing gases with high sulfur dioxide concentrations and very low NOx content, a two-stage plant is proposed as described in DE 1 948 754, in which the elementary sulfur is first burnt under substoichiometric conditions (oxygen debt). Upon passing through a heat exchanger, the produced gases containing sulfur dioxide and sulfur are then subjected to post-combustion with oxygen-containing gases in a further apparatus at about 1000° C. In terms of plant construction, however, this multistage installation is quite complex and hence expensive.